Tuesday, December 13, 2011

Anders Jonas Ångström and the ångström

Anders Jonas Ångström
(1814-1874)

As any good chemist knows, an ångström is equal
to 1x10-10 meters
and is designated by the symbol Å, complete with the little circle on top. I’ve often wondered where the circle came
from,1
but surprisingly until starting this blog had not wondered if the angstrom was
named after a person, especially since it does not follow the metric system
prefixes. Anders Jonas Ångström
was born in Sweden in 1814 and studied physics at the University of Upsala. He didn't leave except for brief sojourns to further science. He was interested in astronomical work, and
studied at the Stockholm Observatory before becoming the observer at the Upsala
Observatory. The Stockholm Academy of Sciences
gave him the job of analyzing the magnetic data obtained by the “Eugéne,” a ship which had
travelled around the world from 1851-1853.
In 1858 he became chair of the department of physics at Upsala
University (I told you he didn’t leave).

Ångström is most known, when he is thought of at all, for his work
in optics which led him to be considered one of the founders of
spectroscopy. His work was primarily with things that gave off light, such as electric sparks. His greatest work was with the solar spectrum, and it is from this work that his fame as the
namesake of the ångström comes. By studying the wavelengths of light emitted by the sun using diffraction gratings, he determined in 1862 that the sun’s atmosphere contained hydrogen. In 1868 he published a book containing a map of the entire visible solar spectrum,
consisting of 1000 lines: Recherches sur le spectre solaire.
Others doing similar work used arbitrary units, but Ångström
used units of ten-billionths of a meter, or at least he thought he did. He discovered that the meter from which he
measured the gratings was too short, and thus all of his calculations were off.2 He began the work to correct it, but died in 1874, before it was finished, leaving his assistant Thalén to finish the job.
A contemporary said that his “work [was] characterized by such accuracy
and completeness as to render it worthy of the highest admiration, to be
regarded as a pattern to all investigators.”3 Although others had attempted to make such
maps, Ångström’s
was the most complete and accurate, and so those who came after him used
his units to describe future measurements, calling them at first Ångström units and then ångströms.

Ångströms, however, are not the best units for describing visible light, since they result in numbers in the thousands, such as 6534. The wavelengths Ångström described are commonly now referred to in nanometers, so the values are only in the hundreds. Chemists, however, have a great affinity for
the ångström,
since it is the perfect unit for describing the length of chemical bonds, which
are on the order of an ångström. So, although the angstrom is no longer used in the field in which it
originated and has been relegated to the status of a “non-SI unit,” it still
finds its uses in chemistry although Ångström and his work have been long
forgotten.

[1] Upon further investigation, the ring
is not a diacritical mark, but an integral part of the letter in Sweden.

[2] Some idea of the complication
of the creation and recalculation of this map can be gathered by looking at the
exercises in Robert
Alexander Houstoun's A Treatise on Light, p. 254.

As a reward for reading the footnotes, here is a link to an argument that Ångström had with a fellow physicist in the Philosophical Magazine: "Observations on Certain Lines of the Solar Spectrum". Note that the fellow who argues against Ångström, Pierre Janssen, doesn't have a unit named after him, but they both gave their names to (separate) lunar craters, and only Janssen has a crater on Mars.